Kirkwood–Buff derived force field for alkali chlorides in simple point charge water

Abstract

Solvated ions are a fundamental constituent of many biological systems. An important class consists of the alkali cations. In particular, potassium (K(+)) is the most abundant ion in the cytoplasm, whereas lithium (Li(+)), rubidium (Rb(+)), and cesium (Cs(+)) are of fundamental physicochemical and medical relevance. A powerful tool to understand ion specificity and cellular systems on a microscopic level is provided by molecular dynamics simulations. Previously, reliable force field parameters for Li(+), K(+), Rb(+), and Cs(+) in aqueous solution have not been available for the simple point charge (SPC) water model widely used in conjunction with the GROMOS force field. We used the Kirkwood-Buff theory to develop force fields for Li(+), K(+), Rb(+), and Cs(+) in SPC water to reproduce experimental data on respective aqueous alkali chloride solutions (LiCl, KCl, RbCl, CsCl). The force field developed reproduces many of the known properties of alkali metal chlorides solutions including densities and partial molar volumes. Our force field is shown to be superior to other common alkali chloride force fields in terms of reproducing the activity derivative, as a prerequisite for a realistic measure of ion-solute association underlying ion-specific phenomena (Hofmeister effects). For lithium and potassium, the ionic radii from cation-water oxygen pair correlation functions and hydration numbers are well reproduced. The force field developed will be useful for modeling physiological conditions and ion-specific phenomena for biomolecular systems.